1. Field of the Invention
The present invention relates to a compressed stream decoding apparatus for decoding plural compression-coded video streams. In particular, the invention relates to a compressed stream decoding apparatus and method capable of decoding plural compressed video streams and outputting plural picture images.
2. Description of Related Art
In digital broadcasting services such as BS digital broadcasting, CS digital broadcasting, and digital terrestrial broadcasting services, an MPEG-2<transport stream has been used. The MPEG-2 transport stream can adopt such a configuration that plural programs are multiplexed in one transport stream.
Up to now, apparatuses for separating plural programs in the MPEG-2 transport stream and decoding plural compressed video streams in the plural programs have been proposed (see Japanese Unexamined Patent Publication No. 11-187396, for example). According to the apparatuses, for example, it is possible to receive the digital broadcasting service to watch or listen to one program while recording the program or another program or to split a display screen into plural areas and display plural programs on the split areas at a time.
FIG. 7 shows the configuration of a conventional compressed stream decoding apparatus capable of decoding plural compressed video streams in plural programs in this way. A compressed stream decoding apparatus 70 receives two transport streams TS1 and TS2 to decode a compressed video stream in one program separated from the transport stream TS1 and output an image from a display output unit 112 as an output image 1. Likewise, a compressed video stream in one program separated from the transport stream TS2 is decoded to output an image from a display output unit 132 as an output image 2.
Further, in the compressed stream decoding apparatus 70, a PLL (phase locked loop) circuit including a comparator 104, a low-pass filter (LPF) 105, a voltage controlled oscillator (VCO) 106, and an STC counter 103 generates a system clock CLK1 synchronous with a PCR (program clock reference) as reference time information extracted from the transport stream TS1. The generated system clock CLK1 is used for determining a decoding timing for a program separated from the transport stream TS2 as well as the transport stream TS1 and for determining an image output timing.
In the compressed stream decoding apparatus 70, if the transport streams TS1 and TS2 are different, different bit rates are used for encoding the respective streams. For example, an STC (system time clock) as a reference clock for encoding and decoding based on the MPEG-2 standard has a tolerance of ±1350 Hz with respect to a reference frequency of 27 MHz. As a result, the STC for encoding programs may vary within tolerance limits in some cases.
In the conventional compressed stream decoding apparatus 70 of FIG. 7, a clock CLK1 after clock recovery from the transport stream TS1 is used by a second compressed video stream processor 72 to decode the transport stream TS2. Thus, the STC for decoding the transport stream TS2 is different from the STC used for encoding the same. In such a case, in the second compressed video stream processor 72, there is a difference between a bit rate for storing video data of a program separated with a DEMUX 121 in an input buffer 127 and a bit rate for decoding the video data in the input buffer 127 with a decoder 128. This difference causes overflow or underflow of the input buffer 127, with the result that the output image 2 is repeated or skipped.
As mentioned above, the conventional compressed stream decoding apparatus has a problem that in the case of decoding plural compressed video streams to output plural picture images, a disturbance occurs in a display image due to skipping/repeating.